The present invention relates generally to optical memories and is more particularly directed to techniques for reading and writing data from and to coherent time-domain optical memories.
Optical memory refers generally to a data storage system that utilizes the properties of a light beam to store data in a storage medium and to retrieve the data from the medium. A light beam has a variety of properties that make it suited for storage and retrieval of data. A light beam may be focused to a very small and precise spatial extent, and this property is exploited in conventional two-dimensional optical memories to store data at geographically defined spatial addresses on the storage medium. Frequency-domain optical memories and coherent time-domain optical memories are examples of other types of optical memories that exploit, in addition, the frequency characteristics, or more generally the spectral characteristics, of the light beam to store and retrieve data at increased storage capacities.
In general, when a light beam impinges on a small spatial region of a storage medium, the medium will absorb the light over a broad band of frequencies. In the frequency-domain method a storage medium is chosen that absorbs incident light at a characteristic frequency or frequencies with an absorption profile that has been inhomogeneously broadened. The spectral properties of suitable materials are well known in the art and need not be described in detail here. To write data onto the storage medium at the small spatial region, a laser beam with very narrow bandwidth compared with the absorption profile of the medium is focused on the region over a sequence of discrete frequencies within the absorption profile. Each such discrete frequency serves as a separate channel for recording a data bit in the spatial region. The beam interacts with the medium at each frequency to produce a gap, or more figuratively to burn a "hole," in the absorption profile depending on whether a data bit is a logic zero or one. In this way, different frequencies carry different data bits in the same small spatial region, which may increase the storage capacity of the medium by a factor as great as one million. This system of optical data storage is disclosed, for example, in U.S. Pat. Nos. 3,896,420 of Szabo and 4,101,976 of Castro et al.
A problem with this frequency-domain method is the slow speed at which data may be written to the medium. If the single channels, i.e., the spectral "holes," have narrow spectral widths, then a large number of channels (as many as one million) may be fit within the width of the absorption profile. But as the single-channel spectral width decreases, the channel access time--that is essentially the length of time needed to write (or read) data in the channel--increases in inverse proportion to the decrease in spectral width. Thus, in this method the great increase in the amount of data that may be written to a spatial region of the medium must be balanced against the proportionate decrease in the data throughput rate because of the necessity of writing the data to one channel at a time.
This problem was addressed by the coherent time-domain method disclosed, for example, in U.S. Pat. No. 4,459,682 of Mossberg. In this method the storage medium is first subjected to an intense, extremely short preparatory pulse of coherent laser radiation. U.S. Pat. No. 4,459,682 refers to the preparatory pulse as the "fixing" pulse, although it will generally be referred to herein as the "write" pulse. Then a coherent laser beam is amplitude-modulated by the data train desired to be stored. U.S. Pat. No. 4,459,682 refers to the modulated beam carrying the data as the "writing" pulse; however, it has become customary to refer to this pulse as the "data pulse" and that terminology will be used herein. The laser beam modulated with the data pulse train is applied to the local spatial region of the storage medium within a characteristic time span from the preparatory write pulse. The characteristic time span is known as the coherence time or the de-phasing time. It provides a measure of the time span over which the atoms in the local spatial region of the storage medium maintain their coherent state after laser excitation. When applied in this manner, the write pulse and the data pulse interfere with one another, and the resulting interference pattern is recorded in the pattern of selective excitations induced in the storage medium. The interference pattern carries within it the entire data pulse train to be written to the local spatial region. The advantage of this time-domain approach over the earlier frequency-domain approach is that the interference pattern, and hence an entire data signal of a great many data bits, may be written to the local spatial region in roughly the same amount of time it would take to write one channel, or a single data bit, in the frequency-domain approach.
The coherent time-domain approach according to known practice is found to be subject to practical limitations on the storage capacity and data throughput, however. According to the above discussion, the write pulse and data pulse must both be initiated and completed within the characteristic de-phasing time of the storage medium. In practice, however, when a long data pulse (comparable to the de-phasing time) is used to record the data on the storage medium, significant distortion is observed when the stored data signal is later read. In fact, distortion in the retrieved data may be observed long before the length of the data pulse reaches even a small fraction of the de-phasing time. Thus, the known methods of coherent time-domain data storage use a data pulse far shorter than the theoretical maximum and fall far short of the theoretical limit on the number of data bits that may be stored in a local spatial region of the storage medium.